I'm so glad someone else noticed that coz it popped into my head when I watched last week's ep :)

Perhaps it is taking a long time to heal because Jared can't simply rest it as a normal person would? Unfortunately, he has to keep using it in the course of his work. It's just a guess on my part.

I find it very hard to figure out the internal timeline of SPN episodes, I'm of the opinon that TPTB keep it deliberately vague.